I recorded a vocal concert in a church. Sounds great, but nobody could find the caretaker beforehand to get the air conditioning shut off. The hum is fairly well masked when people are singing, but as soon as they stop singing, the hum becomes pretty apparent.
 
I can reduce the hum with an equalizer, but it's around 120Hz, so even with the band Q set up as high as it'll go, it erases the basses whenever they're singing a B at 123Hz.

Is there any way to automate the equalizer so that it enables whenever the volume drops below a certain level, and shuts off when the volume exceeds that level? I thought maybe there would be a way to do that via side chaining, but it looks like only gates and compressors support that.

The Sonitus gate can be side-chained, and in a frequency-dependent way.
 
Explanation here https://www.cakewalk.com/Support/Knowledge-Base/2007013060/Sonitus-fxgate
 
It's also possible to manually automate an eq's gain envelope, though you'll need to do that by hand, and another alternative is to go through the track(s) manually and remove the hum in the gaps by automating the track/clip volume envelope. The manual approach is often the least obtrusive because you can refine the volume envelope, or sections to be reduced to silence in a way that is far subtler and less intrusive than relying on an automated gate/expander.

tlw
The Sonitus gate can be side-chained, and in a frequency-dependent way.

 
Right, but I don't actually want to gate the sound (because that kills the echoes); I was hoping I could use a gate to control an EQ effect's enable parameter rather than actually altering the sound itself.

 

Is it basically one big wave/track? If so I could run it through IzoTope RX for you.

1st, the person offering to run it through RX- take the up on it.
2nd, using an expander/gate on a certain frequency range is an interesting idea.  let us know how it goes if you figure it out!

Sanderxpander
Is it basically one big wave/track? If so I could run it through IzoTope RX for you.

Yes, the raw file is just one track. Would IzoTope RX kill the hum without affecting the notes at roughly the same pitch?
 
If so, thanks very much! That would be fantastic.

I'm not a hundred percent sure and I don't have hours to spend on it but it's pretty good and I'd gladly give it a go if you don't mind going through the trouble of sending me the file. Maybe PM me to exchange email addresses if you feel like it?

Here is post about a free plug-in which can perform noise reduction in a similar thread http://forum.cakewalk.com/FindPost/3502845